Conventional organic EL (OLED) display devices including a plurality of organic EL (OLED) elements arranged in a matrix pattern are well known. Among these, attention is especially focused on active-matrix OLED display devices which are expected to become widely used in thin-type display devices. In active-matrix OLED display devices, transistors are provided for respective pixels to control a driving current supplied to each OLED element.
FIG. 1 shows an example of a circuit arrangement for a pixel of a conventional active-matrix OLED display device. In this arrangement, a p-channel TFT (i.e. thin film transistor) 1, used for driving a pixel, has a source connected to a power source PVdd and a drain connected to an anode of an OLED (i.e. organic EL) element 3. A cathode of OLED element 3 is connected to a negative power source CV.
TFT 1 has a gate connected via an auxiliary capacitance C to the power source PVdd on one hand and is connected via an n-channel TFT 2, used for selection, to a data line Data on the other hand. A voltage signal based on pixel data (luminance data) is supplied to the data line Data. TFT 2 has a gate connected to a gate line Gate extending in a horizontal direction.
During display, the gate line Gate is kept at an H level to turn on TFT 2 of a corresponding line. Under this condition, pixel data (i.e. input voltage based on pixel data) is supplied to the data line Data and is stored as electric charge in the auxiliary capacitance C. Thus, the voltage corresponding to the pixel data brings TFT 1 into operation. The current of TFT 1 flows across the OLED element 3.
The light emitted from OLED element 3 is substantially proportional to the current flowing in OLED element 3. In TFT 1, the current begins to flow when a potential difference Vgs, representing a potential difference between the gate of TFT 1 and the power source PVdd, exceeds a predetermined threshold voltage Vth. In view of the above, the pixel data supplied to the data line Data includes a previously included voltage (Vth) which allows the drain current to start flowing at around a black level of the image. Furthermore, the amplitude of an image signal is set to an appropriate value so that a predetermined luminance can be obtained at around a white level.
FIG. 2 shows an example relationship between input voltage (Vgs), luminance of OLED element 3, and current icv flowing in this element (i.e. V-I characteristics). As is apparent from this relationship, the OLED element 3 starts emitting light when the input voltage Vgs reaches the voltage Vth. A predetermined luminance is attained at the input voltage of a white level.
The OLED display device has a display panel including numerous pixels arranged in a matrix pattern. With such a configuration, there is a possibility that, the threshold voltage Vth and the inclination of the V-I characteristics of respective pixels may vary due to manufacturing errors. The light emission from a pixel relative to a data signal (i.e. input voltage) may be different in each pixel, and accordingly this is generally recognized as nonuniformity in the luminance. FIGS. 3A and 3B show differences between two pixels m and n that occur when there is a variation in the threshold voltage Vth or in the inclination of the V-I characteristics. FIG. 3C shows composite differences of two pixels m and n resulting from variations in both the threshold voltage Vth and the inclination of the V-I characteristics. In this manner, when a difference ΔVth in the threshold voltage Vth appears between two pixels, the curve of V-I characteristics shifts by the same amount ΔVth. Furthermore, when the inclination of the V-I characteristics varies between two pixels, their V-I characteristics form the curves different in the inclination from each other. Such a difference in the threshold voltage Vth or in the inclination of the V-I characteristics may occur locally on the display screen.
Therefore, it has been proposed to measure the luminance of each pixel and perform correction for all pixels or only defective pixels based on correction data stored in a memory (refer to Japanese Patent Application Laid-open No. 11-282420, for example)
Furthermore, a technique of dividing the display area into smaller dissected areas and measuring current values in respective areas to obtain an overall tendency, thereby calculating a coefficient for correction of the overall display or of an individual area is also known (refer to U.S. Patent Application Publication 2004/0150592, for example).
However, with the former technique, it is generally difficult to accurately accomplish, within a short time, the measurement of the luminance for numerous pixels, while, with the latter technique, the correctable differences or nonuniformity is limited to the pixels having luminance values continuously changing along the entire display area or pixels having a specific pattern in the vertical or horizontal line.